Booting into the Recovery HD (Command+R at startup) boots your Mac to the OS X Utilities so you can download and reinstall OS X Mountain Lion. Internet Recovery (Command+Option+R) will boot into Apple servers and their OS X Utilities so you download/reinstall your OS X.
You can use either of the above along with an external USB drive and capture the InstalESD.dmg file for the OS so you don't have to download every time.
- Connect an external USB drive with at least 13GB of free space (formated Mac OS Extended-Journal and GUID partitioin map).
- Reboot using one of the above methods.
- From the Mac OS X Utilities screen, click Reinstall OS X.
- Click Continue. You will see the message, “To download and restore Mac OS X, your computer’s eligibility will be verified with Apple”. Click Continue, then Agree twice.
- Select the external drive to install Lion onto.
- IMPORTANT: Monitor the download’s progress. As the progress bar gets near the end, get ready, because once the status reads About 0 seconds remaining, the progress bar will disappear, the installer will spend a minute or two cleaning up, and then your Mac will restart. As soon as the screen goes dark, unplug your external drive.
- Once your Mac has booted from its normal startup volume, reconnect the external drive.
- Open the external drive, and you’ll find a folder called Mac OS X Install Data. The important file is the one called InstallESD.dmg, just under 4GB in size, which is a bootable disk image containing the Mountain Lion installer. (If the Mac OS X Install Data folder has a “no access” icon, select the external drive in the Finder, choose File -> Get Info, and expand the Sharing & Permissions folder in the Info window. Click the padlock icon at the bottom of the window, provide an admin-level username and password, and then uncheck the box next to Ignore Ownership On This Volume.).
- Copy the InstallESD.dmg disk image to your Mac’s internal drive.
- Launch Disk Utility
- Insert an 8GB USB thumb drive. Format it to Mac OS Extended-Journal and GUID partitioin map.
- Click the Restore tab.
- Drag the InstallESD.dmg file to the Source box and the thumb drive's single partition to the Destination box. Click the Restore button.
- Test the installer thumb drive. Note: If you have an issue with booting to the thumb drive, do a PRAM reset and try again. You should be fine.